Cold electrode pulse lamp structure



Aug. 30, 1955 E. B. NOEL COLD ELECTRODE PULSE LAMP STRUCTURE Filed March22, 1950 4 M Bx: MQKX. fl Z 5 WwQWAm Invewbor Edward B. Noel,

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United States Patent 0 COLD ELECTRODE PULSE LAMP STRUCTURE Edward B.Noel, Cleveland Heights, Ohio, assignor to General Electric Company, acorporation of New York Application March 22, 1950, Serial No. 151,098 6Claims. (Cl. 313-185) My invention relates to gaseous electric dischargelamps generally, and more particularly to electrode structures for suchlamps designed for repetitive pulse flashing.

In connection with the transmission of standard motion picture film overtelevision systems, there has lately arisen a demand for a flash lampcapable of successfully withstanding repeated applications oftremendously high energy loadings, and on which very stringentrequirements of constancy in operation have been imposed.

A system in which such a lamp is utilized is described in my co-pendingapplication Serial No. 151,097 filed March 22, 1950, entitled ThermionicElectrode Pulse -Lamp Structure, and assigned to the assignee of thepresent invention. In film television it is necessary to reconcile theconflicting frame and sweep synchronization rates of standard film andstandard television, film pulldown being at the rate of 24 frames persecond, and television sweep being at the rate of 60 sweeps per second.In this particular film televising system these requirements arereconciled by providing a source of light which is pulsed alternatelythree times and two times for successive film frames. In this manner thefilm pull-down and television sweep are maintained in step and re-occursimultaneously at every of a second, in which interval of time two filmframes and five television sweeps occur.

In the aforementioned application a lamp structure for such a system isdisclosed and claimed in which small thermionic electrodes composed ofhelixes of refractory metal wound around a refractory metal rod andcontaining a sliver of thorium are utilized. With electrodes of thattype the diificulty was encountered that small deposits of tungsten,commonly known as tree growths, occurred on the electrodes; thesegrowths, and also the forward face of the cathode, glowed in between theflashes of light, and produced a direct or residual component of lightwhich affected the televised image by producing so-called travel ghosts.This problem was solved by constructing the thermionic cathode with asleeve of a highly refractory insulating material such as quartz oralumina so that the tree growths occurred inside the sleeve and thedirect component of light was eliminated.

The present invention is concerned with a lamp designed for essentiallysimilar applications in which solid massive electrodes, which operate ata relatively low temperature, are used. The presently commonly acceptedtheory, though it is to be understood that it is not stated here as anuncontrovertible fact, is that the emission from these electrodes is duepartly to field emission and partly to the formation of a hot spot. Suchan electrode structure, for instance, is described in my Patent2,459,579 assigned to the same assignee as the present invention. Theseelectrodes comprise a massive body portion and an electron emissive tipor conical portion containing a small proportion of barium.

When lamps containing this type of electrode are used for repetitivepulse flashing, it has been found that the arc does not stay on the apexof the conical portion of the cathode but wanders from one side to theother on' the cone. This appears to be caused by the continuousvaporization of the barium under the cathode spot. This may be quiteobjectionable, particularly when the lamp' is used in a film televisingsystem in which a wide angle light-condensing system is utilized tofocus the light on the film. The wandering of the arc causes rapidspasmodic fluctuations in the brilliancy of the televised image whichproduces considerable discomfort to the ob': server. This appears to becaused by the shifting of the arc in and out of the focal plane of thecondensing system, depending upon whether it strikes on the tip of theconical electrode or on one side thereof.

Accordingly, it is an object of this invention to pro vide a new andimproved lamp and electrode structure for preventing arc wander.

Another object of this invention is to provide an improved lamp forrepetitive pulse flashing which assures an arc of constant intensityduring successive flashes, and which is accurately maintained in a focalplane.

In accordance with the present invention these objects are achieved bymeans of a lamp containing a massive cathode of the hot cathode spot andfield emission type. Preferably, the cathode has a conical tip, and overthis tip a close-fitting conical cap of a highly refractory in-'sulating material is fitted. The insulating cap has a small perforationin its apex through which the arc is con strained to enter, so that theexternal portion of the arc is stabilized, even though the hot spot mayshift under the insulating cap.

For further objects and advantages and for a better understanding of theinvention attention is now directed to the following description andaccompanying drawing. The features of the invention believed to be novelwill be more particularly pointed out in the appended claims.

In the drawing:

Fig. 1 is an elevation view of a gaseous electric dis-' charge lampembodying my invention.

Fig. 2 is an exploded sectional elevation on an enlarged scale of acathode structure embodying my invention.

Fig. 3 is an elevation view of a cathode structure em= bodying apreferred form of my invention.

Fig. 4 is an exploded sectional elevation of the cath ode structure ofFig. 3.

Fig. 5 is an elevation view of a pair of electrodes lo-v cated inoperative relation for the certain features of my invention.

Referring to Fig. l, the lamp 1 shown therein comprises a tubularlight-transmitting bulb 2 which may be made of quartz or of a hard glasshaving a high melting point and a low coeflicient of expansion. The bulbis provided with tubular appendages or necks, to each of which is sealeda terminal structure comprising a purpose of illustrating heavy rigidlead-in conductor in the form of a post or prong 3 of steel havingbrazed thereto a thin metal ferrule 4, the rim of which is fused intothe end of the neck 5. The ferrules 4 may be made of an alloy capable ofhermetically scaling to a glass bulb, for example, a nickel-iron ornickel-iron-cobalt alloy, as is well known.

At the center of the bulb is an arc gap, preferably about 3 to 4 mms.wide, between two massive electrodes 6 and 7 which are suitably afiixed,either by brazing or spot welding, to inward extensions of the posts 3,3'. Where the lamp is to be utilized on alternating current bothelectrodes are made identically to the cathode 7 now to be described.

The cathode 7, whose construction may be more readily visualized withreference to Fig. 2, comprises a cylindrical body portion 8 also made ofcold rolled steel and having on its upper circular surface a smallprojection or boss 9. The conical tip portion 10 of the electrodeis-composed of a combination of refractory and alkaline earth metal,preferably a tungsten-nickel-harium alloy which may have a compositionof approximately 89 per cent tungsten, 10 per cent nickel, and 1 percent barium. The conical tip portion 10 is joined to the body portion 8by a suitable electric welding process which in its essence consists ofpassing a heavy current through the tip and body portion simultaneouslywith the application of axial pressure to force both portions together,the boss 9 melting and flowing to form the weld. A suitableproportioning for electrode 7 is a body portion inch in diameter andabout 1 inch long, with the conical tip forming a 60 degree angle at itsapex.

Over the cathode 7, and proportioned to fit quite snugly thereon, Iprovide a cover 11 composed of a refractory insulating substance andshaped to conform to the dimensions of the electrode. Cover 11 comprisesa cylindrical sleeve portion 12 and a conical cap portion 13 which isperforated at its apex by a small hole 14 which may be approximately 50mils in diameter. The quartz cover is slipped over the electrode andheld in position by means of a wire ring 15 fitting around the expandedbase portion thereof and spot welded to the extension of the bindingpost 3.

The bulb 1 contains a suitable ionizable medium such as a filling ofargon, krypton, xenon, or mixtures of those gases with hydrogen. Apresently preferred filling is xenon at a pressure of 600 mm. of mercuryin a bulb 1% inch in diameter and about 3 inches long. The bulb with itsfilling at near atmospheric pressure is tipped off or sealed at thetabulation 12 by methods well known in the art.

When lamp 1 is operated as a repetitive pulsing source, I have foundthat the quartz cover substantially reduces or eliminates the tendencyof the arc to wander from side to side of the conical tip of thecathode. Referring to Fig. 5, the are always enters through the hole 14along the axis XX, and strikes a hot spot on the conical tip portion1%). In operation, the sharp apex of tip 10 slowly disintegrates and acrater 17 is formed under the hole 14, as may be seen in dotted outlinein Fig. 5. Quite surprisingly, the tendency of the arc to cut or burnaway the edges of the hole 14 is much less than would be expectedbecause of the fact that the arc does not penetrate in the crack betweenthe quartz and the conical tip but, instead, strikes within the crater.This appears to be due to the fact that the width or lateral dimensionof the crack between the cover and the electrode is quite small, so thatthe wall losses therein are too high to permit substantial ionization,and the arc cannot creep into the crack.

Referring to Figs. 3 and 4, there is shown a preferred structure whereinthe cover is restricted to the conical tip portion 16 of the electrode.The cover in this case consists only of a conical cap 13 with the holeor perforation 14 at its apex and held in place by means of springfingers 19 which are welded to the sides of the body portion 8 of theelectrode. As may be seen in the drawing, the conical cap 18 covers onlythe conical tip portion 10 of the electrode, being secured in place bythe fingers 19 extending into clamping engagement therewith. Thisstructure requires less quartz so that it is more economical, but itsoperation is identical to that described with reference to Fig. 2.

Although the lamp of Fig. 1 shows only electrode 7 provided with aquartz cover, it will be understood that when such a lamp is to be usedon alternating current both electrodes would be made identical toelectrode 7 since each would act alternately as cathode and as anode.However, when the lamp is to be used with unidirectional pulsed current,i have found that there is little tendency for the arc to wander overthe tip of the anode, so that a quartz cover therefor is unnecessary andneed be provided over the cathode only.

Although I have described the electrodes as having a 60 conical tip, itis readily seen that the invention is likewise applicable to electrodesof this type having a tapered end, whether curved or pointed, of thegeneral configuration which has been described. Likewise, otherrefractory insulating materials, such as alumina, beryllia, magnesia,thoria and zirconia, may be used for the insulating cap instead of thequartz which has been described.

The lamps in accordance with the present invention, and also those inaccordance with my co-pending application entitled Thermionic ElectrodePulse Lamp Structure, have, for the first time, made practical thetelevision of film by means of a pulsed light system, as described inthat application. Previously it had been impossible to produce atelevised image sufficiently free from objectionable flicker to renderthis system competitive with the more conventional systems in which thelight from a constant source is mechanically interrupted by a shutter.My present invention has permitted a reversal of this situation andenables the present system to compete on very favorable terms with theconventional shutter systerns.

While certain specific embodiments have been shown and described, itwill, of course, be understood that various modifications as to theshape, configuration and composition of the electrodes, as to the gasfilling of the envelope, and as to the configuration and composition ofthe insulating cover, to mention but a few, may be made withoutdeparting from the invention. The appended claims are therefore intendedto cover any such modifications coming Within the true spirit and scopeof the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. A high pressure electric discharge lamp comprising alight-transmitting envelope containing an ionizable gaseous filling, apair of massive electrodes oppositely mounted in alignment within saidenvelope and defining a high intensity are path, at least one of saidelectrodes having a tapered tip portion comprising refractory metal andalkaline earth metal, and a refractory insulating cover fitting snuglyover said tapered tip portion and having a small perforation at its apexfor stabilizing the path of the are occurring between said electrodes.

2. A high pressure electric discharge lamp comprising alight'transmitting envelope containing an ionizablc gaseous filling, apair of massive electrodes oppositely mounted in alignment within saidenvelope and defining an arc path less than the transverse diameter ofthe envelope part surrounding them, at least one of said electrodeshaving a conical tip portion comprising refractory metal and alkalineearth metal, and a refractory insulating cover fitting snugly over saidconical tip portion and having a small perforation at its apex forstabilizing the path of the are occurring between said electrodes.

3. An electric discharge lamp comprising a light-transmitting envelopecontaining a filling of an inert gas at near atmospheric pressure and apair of massive electrodes oppositely mounted in alignment within saidenvelope by lead-in conductors and defining an arc path less than thetransverse diameter of the envelope part surrounding them, at least oneof said electrodes having a conical tip portion comprising refractorymetal and a small percentage of alkaline earth metal, and a refractoryinsulating cover fitting snugly over said conical tip, and having asmall perforation at its apex for stabilizing the path of the areoccurring between said electrodes by forcing the arc to enter throughsaid perforation, immatcrially of the place of striking on said conicaltip portion.

4. An electric arc discharge lamp comprising a lighttransmittingenvelope containing a filling of an inert gas at near atmosphericpressure and a pair of massive electrodes oppositely mounted inalignment within said envelope for defining a high intensity are pathshorter than the transverse diameter of the envelope part surrounding 7them, at least one of said electrodes having a conical tip portioncomprising refractory metal and a small per centage of alkaline earthmetal, and a refractory insulating conical cover conforming to andfitting snugly over said conical tip, said cover having a smallperforation at its apex for stabilizing the path of the are occurringbetween said electrodes by forcing said arc to enter through saidperforation in forming a cathode spot on said conical tip portion, sothat said are remains stable in said path independently of the shiftingof said cathode spot on said portion.

5. An electric arc discharge lamp comprising a lighttransmittingenvelope containing a gas, at near atmospheric pressure, of the groupincluding argon, krypton, xenon, hydrogen and mixtures thereof, and apair of massive electrodes oppositely mounted in alignment within saidenvelope for defining a high intensity are path shorter than thetransverse diameter of the envelope part surrounding them, at least oneof said electrodes serving as a cathode having a solid conical tipportion comprising refractory metal and a small percentage of alkalineearth metal in order to operate partly through field emission with theformation of a cathode spot having a tendency to wander, and a conicalcap of a highly refractory insulating material conforming to and fittingsnugly over said conical tip portion, and having a small perforation atits apex for stabilizing the path of the art: occurring between saidelectrodes by forcing said arc to enter through said perforation informing a cathode spot on said conical tip portion, so that said arcremains stable in said path independently of the shifting of saidcathode spot on said portion.

6. An electrode for electric discharge lamps comprising a massivecylindrical body portion of metal having a comparatively high workfunction and terminating at one end in a conical tip portion ofrefractory metal and emissive material having a comparatively low workfunction, and a conical cap of refractory insulating material fittedsnugly over and covering only the said conical tip portion and having asmall perforation at its apex exposing only the apex of said conical tipportion, and spring finger members secured to the sides of said bodyportion and extending therefrom into clamping engagement with said capto hold it against said conical tip portion.

References Cited in the file of this patent UNITED STATES PATENTS1,634,201 Massolle et al June 28, 1927 1,808,826 Teasdale June 9, 19311,809,447 Howard June 9, 1931 1,879,740 Gross Sept. 27, 1932 1,932,025Thomas Oct. 24, 1933 2,087,735 Pirani July 20, 1937 2,135,661 Hagen eta1. Nov. 8, 1938 2,153,009 Scott Apr. 4, 1939 2,229,329 Kaspar Jan. 21,1941 2,238,277 Miller Apr. 5, 1941 2,298,965 Pellmann Oct. 13, 19422,375,808 Miller May 15, 1945 2,459,579 Noel Jan. 18, 1949

